Ultrasound probes with acoustical lenses are often susceptible to fluid ingress. Typically, the acoustic stack within the lens is protected by an insulation layer of material which forms a moisture barrier to mitigate this problem. The insulation layer is often placed on an outer surface of the acoustical lens, where it is subject to wear, or directly on the acoustic stack itself where it can affect acoustical characteristics of the acoustic stack. The insulation layer can also be embedded within the acoustical lens itself so it is internal to a mechanically protected by the lens. A consequence of this arrangement is the introduction of an interface between the inner and outer portions of the acoustical lens which presents itself as a reflecting surface to ultrasound waves within the acoustical lens. Such an interface can give rise to unwanted reflections of ultrasound waves that may be of sufficient magnitude to be observable in the ultrasound image as artifacts, adversely affecting image quality. Therefore, it is desirable to design the interface which minimizes the magnitude and coherence of these reflections.
Ultrasound probes that employ acoustical lenses with multiple materials of different kind in two or more regions are also subject to the problem of internal interface reflections, since the boundaries between different materials are themselves reflecting surfaces capable of producing acoustic reflections which can lead to image artifacts. Choosing lens materials having specific acoustic properties has been common practice, e.g. for impedance matching purposes, but this restricts the number of lens materials suitable for use, and complicates the implementation of some of the technologies proposed in the references below.
WO 2010/086779 A2 describes a two-part fluid acoustical lens system with an interface between the fluids. Fluids with specific properties are chosen to reduce reflections only at certain incidence angles.
Other ultrasound probes with multiple acoustical lens systems have been proposed and each contains interfaces between acoustical lens materials and layers that present themselves as reflective surfaces.
U.S. Pat. No. 5,577,507 describes acoustical lens systems with an outer material of improved durability.
WO 2008/051473 A2 describes acoustical lens systems where materials are chosen to apodize or shape ultrasound beams to reduce sidelobes.
US 2011/0071396 A1 describes an acoustical lens with an internal electrical conducting surface for CMUT probes. None deal with the problem of internal acoustical lens reflections created in these structures.